I have to design a PID controller for a differential drive robot.

• The hardware board consists of a AVR atmega168 running at 16mhz.
• Driven by two 24v scooter motors with 256 CPR encoders
• The motors spin roughly at about 4700 RPM and has a 51.56:1 gear ratio

So far I can read the encoder direction and the tick count. But I have no idea how to convert this to speed. And eventually a PID controller. I also want to be able to rotate the robot at a given angle. Can someone please help me to get started ?

Currently I am having problems figuring out a suitable timer calculation for this.